The Science of HVAC Efficiency Ratings
Specific Formulas Clearly Define System Precision
Customers purchasing heating or cooling equipment typically want to know how energy efficient their new units will be. Contractors also want to be sure that the systems they are selling perform as advertised, which is why manufacturers usually rely on an unbiased third-party verifier, such as the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), to independently measure the performance — or energy efficiency — of their HVAC equipment.
Most in the industry are already familiar with many of the terms used to describe the energy efficiency of a unit — SEER, AFUE, EER, COP, etc. But other measurements are starting to be used as well, such as CEF, IEER, SCHE, and SCOP, which have been developed by AHRI, the U.S. Department of Energy (DOE), U.S. Environmental Protection Agency (EPA), and other groups. Manufacturers say these terms are useful to know when determining the amount of energy used by certain systems.
Heat Pump Ratings
The appropriate rating acronym depends on the type of system that is being considered. For residential gas furnaces, one term is typically used in all instances: AFUE, which stands for annual fuel utilization efficiency. AFUE is a measure of how efficient the furnace is in converting the energy in its fuel to heat over the course of a typical year. Per the DOE, AFUE is the ratio of annual heat output of the furnace compared to the total annual fossil fuel energy consumed by a furnace. For example, an AFUE of 90 percent means that 90 percent of the energy in the fuel becomes heat for the home and the other 10 percent escapes up the chimney or elsewhere.
Air-source electric heat pumps are a little more complicated, as they have several different energy-efficiency ratings that can be used to describe their performance. The most common, seasonal energy efficiency ratio, or SEER, measures cooling efficiency. It is calculated as the total heat removed from the conditioned space during the annual cooling season, expressed in Btu, divided by the total electrical energy consumed by the heat pump during the same season, expressed in watt-hours. Another cooling efficiency measurement is EER [energy efficiency ratio], which is the ratio of the heat removed (in Btuh) to the electricity required (in watts) to run the unit.
A heat pump’s heating efficiency is indicated by the HSPF [heating season performance factor], which is the total space heating required during the heating season, expressed in Btu, divided by the total electrical energy consumed by the heat pump system during the same season, expressed in watt-hours. And yet another term that can be used to describe a heat pump’s efficiency is COP, or coefficient of performance, which is the ratio of either heat removed (for cooling) or heat provided (for heating) in Btu per Btu of energy input.
In short, SEER and HSPF take into account the seasonal temperature and humidity variability under which heating and air conditioning systems operate, said Matt Lattanzi, director of product management, Nordyne. “EER and COP also measure cooling and heating efficiencies respectively, but these measurements are at one operating point and do not account for seasonal variations in temperature and humidity.”
When it comes to talking with customers about the efficiency of a heat pump, Lattanzi recommends that contractors stick with SEER and HSPF. “These metrics are visible to consumers via the Energy Guide label; EER and COP are not. That’s because the DOE can only regulate one metric for heating and one for cooling, and HSPF and SEER are those chosen metrics. By law, HVAC manufacturers can only promote these same metrics, which is why we talk about SEER and HSPF for heat pumps and not EER and COP. While important, EER and COP are not regulated and do not have established minimums, which can create confusion for homeowners.”
A Comparative Tool
As defined in AHRI Guideline V–2011, if a system includes an air-to-air energy recovery ventilation (ERV) component with unitary equipment (packaged air conditioners, heat pumps, heating-only units, etc.), the energy efficiency at a design operating point is calculated by the CEF [combined efficiency factor]. CEF is a similar calculation to EER, but CEF outlines a method to sum the performance of individual components, said Scott Laurila, product manager, tempered air products, Greenheck Fan Corp.
“The CEF calculation combines the cooling capacity of the ERV component with the mechanical cooling capacity of the unitary equipment,” said Laurila. “Dividing the total net cooling delivered by the total electrical power consumed of the system results in a CEF. CEF can also calculate system efficiency during heating operation by determining the total heating delivered versus the total electrical power consumed.”
An example CEF calculation for an ERV matched with unitary air conditioning equipment is as follows:
CEFCooling = (Net Cooling CapacityERV + Net Cooling CapacityUnitary) ÷
(Electric Power ConsumedERV + Electric Power ConsumedUnitary)
Although the calculations in AHRI Guideline V require the ERV component to be certified, some of the data required to calculate CEF are not certified; therefore, CEF should be used as a comparative tool between different combinations of ERV components with unitary equipment, said Laurila.
“For example, one combination of an ERV component and unitary equipment may provide more net cooling, but the pressure drop of the system could require higher fan motor brake horsepower and electrical power consumption,” he noted. “This leaves the potential for a less effective ERV component with a lower pressure drop to have a higher CEF rating and ultimately resulting in a more efficient system for the end user.”
CEF is helpful in providing contractors, engineers, and end users a means to evaluate the energy efficiency of a system, said Laurila. “The concept of subsystem ratings like CEF may become a bigger part of energy codes and standards in the future, as the advancements in component technology — such as compressors, motors, fans, and ERV devices — may not be enough to achieve our energy-efficiency goals.”
New Seasonal Measurement
A newly developed measurement that evaluates the efficiency of a unitary air conditioning or heat pump system on a seasonal basis is the IEER [integrated energy efficiency ratio]. While often used for water- or air-cooled chillers and variable-refrigerant flow (VRF) split systems, it can also be applied to unitary packaged units and split systems over 5 tons, said Mike Ray, senior product manager, commercial rooftops, Lennox Commercial.
“IEER is best suited for chillers and VRF systems because these systems generally utilize multiple air handlers (or fan coils) as part of the system,” said Ray. “During operation, there will be times when it is not necessary to operate all of the air handlers (fan coils), and under these conditions, the system’s efficiency is higher. The IEER calculation evaluates the system operation at different specific conditions. The rating was developed as a way to compare operating efficiencies of different systems under the same conditions.”
IEER is calculated as follows:
IEER = (0.02 A) + (0.617 B) + (0.238 C) + (0.125 D)
A = EER at 100 percent net capacity at AHRI standard condition (95˚F)
B = EER at 75 percent net capacity and reduced ambient
C = EER at 50 percent net capacity and reduced ambient
D = EER at 25 percent net capacity and reduced ambient
While the IEER is a helpful value to know, the current certified efficiency measurement, EER, is usually the most appropriate for contractors to use, said Ray. “EER is important to the end user and utilities in reducing demand during the peak hours of the year; however, many state and utility incentive programs have recently incorporated both EER and IEER into their rebate offerings. In locations where there are incentives for both ratings, we encourage contractors to communicate the EER and IEER values to their customers.”
Other Ratings of Note
Another measurement used for VRF systems is SCHE [simultaneous cooling and heating efficiency], which is a tested energy metric that is defined by AHRI Standard 1230 as the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in heat recovery mode. It is a metric that applies only to units or systems with energy recovery features, said Jill Hootman, manager of commercial unitary product planning, Trane, a brand of Ingersoll Rand. “SCHE only applies to VRF heat recovery systems (capable of simultaneous heating and cooling); it does not apply to heat pump VRF systems.”
It is important to calculate SCHE when designing certain VRF systems, said Hootman, but it is also important to consider EER, which measures full-load energy performance at a defined temperature, and IEER, which is a part-load metric that portrays the energy efficiency of the system when only part of the system is needed. “IEER is a measure of VRF efficiency in cooling mode and applies to both heat recovery and heat pump systems.”
Like VRF systems, computer and data processing room unitary air conditioners also have a specific energy-efficiency rating. Per ASHRAE Standard 127-2007, SCOP [sensible coefficient of performance] is defined as the ratio of the net sensible capacity of the unit in watts divided by the power input in watts.
“The difference between SCOP and other ratings, such as EER or COP, is that SCOP is based on the cooling unit’s sensible capacity,” said Dave Kelley, director, application engineering, Liebert Precision Cooling, Emerson Network Power. “Computer room/data center loads are primarily sensible, and units designed for those applications are optimized around sensible capacity, not total capacity.”
Kelley noted that ASHRAE Standard 127-2012 changed the term SCOP to NSenCOP, although their definitions are identical, and AHRI Standard 1360P-2013 also uses the term NSenCOP with the same definition.
With a veritable alphabet soup of acronyms now available to describe the energy efficiency of HVAC equipment, contractors have all the information necessary to educate their customers about how their systems will perform when installed correctly.
Publication date: 12/30/2013